Well, normally I point out Unicron nomming a planet, but somehow that doesn't have the impact...
...when I thought of the Supernova Bomb first.
In his third Hitchhiker's Guide book, Douglas Adams depicted a special sort of bomb, relatively small device which - upon detonation - connects itself to all suns and stars in the universe by hyperspace and thereby sets them all off at once. This was devised by the super computer of the Krittetmen, Hactar, who did not create a working model the first time, but it was revealed that he DID, in fact, make one later on.
Yes, you got it. You know how Matt Smith's TARDIS exploded and took the universe with it? That's what would happen, only without an emergency time-field holding ANYTHING together. All that makes the universe 'The Universe' as we know it dies out in a second, in the biggest bang since the first one.
Does anyone have a better one?